Both are great cards. I have the gigabyte so I cant speak for the MSI, but people with the MSI seem to enjoy it.
They both seem to overclock very similarly. I have mine OC'ed to 1500mhz. Most, if not every, MSI and gigabyte model should hit right around there. As for cooling, my gigabyte hits around 75 degrees max when overclocked and running unigine heaven and it stays very quiet. Under load both the MSI and gigabyte stay fairly quiet, but the MSI does have the passive cooling when idle. The gigabyte comes with a backplate while the MSI doesn't. If you have a blue build, the blue led on the gigabyte will look sexy, but you can turn it off if you don't like it. I have to say I'm very happy with my gigabyte. Hopefully someone with an MSI will give their input.